1. Field of the Invention
The invention relates to an apparatus for determining the quantity of particles present in a container, having a sensor device which is movably mounted in the container, which is in contact with the particles and, when the filling height in the container falls below a predetermined value, which actuates an electrical switch member to deliver a signal to an indicator instrument.
The device is suitable, inter alia, for use in electrophotographic copying machines which have storage containers filled with pulverulent toner for replenishing the developer mixture.
2. The Prior Art
As is known, the development of a latent image on an information carrier takes place in such a way that a developer mixture, consisting of toner particles and carrier particles, is brought into contact with the electro-static charge image. On mixing with the carrier particles, the toner particles obtain a tribo-electric charge of defined polarity. The electric field emanating from the charge image attracts the toner particles and detaches them from the carrier particles. Due to the depletion, thus caused, of toner particles in the developer mixture, toner must continuously be supplied to the developer mixture in order to hold the total toner concentration in the developer mixture substantially constant. There is thus a demand for a device for detecting the filling height of toner in a storage container, which device timely indicates that the toner particles are consumed or the filling height within the storage container has fallen below a given mark, so that the operating personnel refill the storage container with toner, and operation of the copying machine with uniform image quality of the developed images is thus ensured.
A number of level-measuring devices for detecting the filling height of toner particles in a toner stock container is known, among which, inter alia, an electrical switching arrangement may be mentioned; the arrangement responds when the filling height in the storage container falls below a given value, emitting a signal to an indicator instrument in a remote position. It is a disadvantage here that, because of their small size, the toner particles can penetrate into the switch or into the switch elements and can thus cause interference with the generation of the signal.
German published application DE-OS No. 2,261,253 discloses a level-measuring device in which a sensor device movably mounted in a container is in contact with the toner particles and, when there is a distance between the toner particles and the sensor device, can be moved from a first into a second position which indicates that the level of the toner particles present in the storage container has fallen below that predetermined for optimum operation of the copying machine. The sensor device consists of a bar magnet, which is fitted on a rotatably mounted axle in the interior of the storage container, and of two further bar magnets which are fixed to the inner walls of the stock container. In the filled state of the storage container, the bar magnet joined to the axle is immersed in the toner and its longitudinal axis extends approximately at a right angle to the longitudinal axes of the two bar magnets which are mutually aligned. As soon as the toner stock in the stock container has falled sufficiently that the bar magnet provided on the axle is released, the latter executes a rotation by 90.degree. under the influence of the magnetic field lines of the two other bar magnets and comes to lie in one plane with these two bar magnets. In other words, this means that the two magnet arrangements are designed in such a way that together they generate a magnetic field which exerts a torque on the axle. This magnetic torque effects the rotation of the axle from the first into the second position when the toner present in the stock container falls below a predetermined minimum level. On rotation from the first into the second position, an electrical switch is actuated to generate an electrical signal which triggers an indicator device.
Due to the use of at least three bar magnets, this known device is relatively expensive to manufacture and does not provide resetting of the measuring device into the starting position before replenishing with toner. This means that, before toner is filled into the storage container, the bar magnet seated on the axle must be manually brought into its original position, that is to say, the first position, in which its longitudinal axis extends perpendicular to the mutually aligned longitudinal axes of the two other bar magnets, thereby ensuring a fault-free indication of the filling level.